1. Field of the Invention
The present invention relates to a pixel circuit, and more particularly, to a data voltage current-drive OLED pixel circuit. The circuit establishes a threshold voltage of a current drive transistor operating in saturation across a storage capacitor. Thereafter, the circuit writes a data voltage into the storage capacitor for controlling current through the OLED.
2. Description of the Prior Art
One method to achieve large size OLED (organic light emitting diode) displays is to use an active matrix thin film transistor (TFT) backplane. An active matrix consists of an array of rows and columns of pixels each having an active device such as a transistor. Row lines driven by row drivers are sequentially selected, one row line at a time, from top to bottom, while data for a selected row is presented on data lines or column lines by data or column drivers. The selected row turns on a pixel transistor that connects the data, typically in the form of a voltage, to a pixel circuit. The row lines are connected to gates of the pixel transistors and are often referred to as gatelines. Typically, pixel storage capacitors are used to store the data voltage. Leakage currents require that the pixel voltages be refreshed or updated. The refresh or frame rate for driving OLEDs is typically 60 Hz. The maximum time available for writing data into each row is tf/n where tf is frame time and n is the number of rows in the display.
Some manufacturers are using amorphous silicon (a-si), as opposed to polysilicon (p-si), to make active matrix OLED displays. In order to achieve sufficient luminance uniformity, OLED pixels are driven with current and not voltage. Amorphous silicon does not have complimentary devices as do polysilicon or crystalline silicon devices. Only n-type field effect transistors (NFETs) are available in amorphous silicon. Due to the manner in which the OLEDs are usually fabricated, i.e., having a common cathode for all pixels in the display, it is not normally possible to drive the OLEDs with a current source comprised of NFETs.
In typical active matrix addressing, voltage signals are written into each pixel to control the pixel luminance. The mobility and stability of amorphous silicon is suitable for driving twisted nematic liquid crystal, which is electrically similar to a small capacitive load and with which a data voltage is applied with a duty cycle in the range of 0.001% to 1%. However, for driving OLEDs requiring nearly continuous current for operation, the amorphous silicon operating voltages are non-zero for a substantially larger percentage of the time (duty cycles up to 100%). The higher voltage-time multiplier severely stresses the TFT. In particular, a gate to source voltage stress causes a threshold voltage to vary due to trapped charge at a semiconductor (a-si or p-si) gate insulator interface or in the gate insulator, and other effects such as creation of defect states and molecular bond breakage at the gate insulator-to-semiconductor interface and in a semiconductor layer. As the TFT's threshold voltage varies, current though the TFT will varies. As the current varies so does the OLED brightness since the OLED light output is proportional to current. A typical human observer can detect a pixel to pixel light output variation of as little as 1%, however, a level of 5% luminance variation is typically considered as being unacceptable. AC voltages on TFT terminals tend to minimize effects of trapped charge and can prolong TFT lifetimes.
IBM Corporation, the assignee of the present application, has considered a-Si TFT OLED current drive pixel circuit having three transistor pixel circuits that use current to write the pixel OLED current. The pixel circuits eliminated any dependence of threshold voltage on the OLED current. The pixel current can sink or source current to the OLED.
Since current is either sourced into or sunk out of the data or column line, the pixel circuits previously disclosed may not be suitable for high format displays. As the display format increases, the number of rows increase, thus increasing column line capacitance. To obtain a wide range of grey levels, the pixel current will need to vary between two and three orders of magnitude. The lower pixel currents may not be able to charge the column line in a line time due to the large capacitance. Higher level currents can be written and for a given luminance the OLED ‘on’ time can be reduced proportionately. However, the higher currents require higher voltage, and thus cause higher stress on the TFT. The higher currents also increase power supply voltage drops and current return voltage drops. At some point with increasing display format, this approach may not be practical. In addition, current source or sink drivers for active matrix organic light emitting diodes (AMOLEDs) are not presently commercially available.
A problem is that although voltage data drivers are readily available, there are no amorphous silicon pixel circuits that can convert the voltage data to current for driving an OLED having a common cathode, without a threshold voltage dependence.
Prior inventors, for example, see U.S. Pat. No. 5,552,678 to Tang et al., have attempted to solve problems of OLED degradation. When a constant voltage is applied, progressively lower current densities result. Lower current densities result in lower levels of light output with a constant applied voltage. Tang et al. incorporates an AC drive scheme of OLEDs, and claims that by applying an alternating voltage across the anode and cathode improves the stability and operating performance of the OLED.
A threshold voltage compensated current source pixel circuit using voltage data and polysilicon PMOS transistors has been described by R. M. A. Dawson et al., “The Impact of the Transient Response of Organic Light Emitting Diodes on the Design of Active Matrix OLED Displays”, IEDM, p 875–878, 1998. The circuit incorporates 4 PMOS transistors and two storage capacitors. The circuit requires custom designed row drivers and the circuit does not appear to be suited for high-resolution displays.
A current writing amorphous silicon pixel circuit has been described by Yi He, et al., “Current Source a-Si:H Thin-Film Transistor Circuit for Active Matrix Organic Light-Emitting Displays”, IEEE Electron Device Letters, Vol. 21, No. 12, p 590–592, December 2000. The circuit incorporates four transistors and a storage capacitor. The circuit requires custom design current data line drivers and the circuit dissipates a substantial amount of power as it incorporates two transistors in series to source current.